Aerosol mass-size distributions at a tropical coastal environment: response to mesoscale and synoptic processes

Abstract

Regular measurements of total mass concentration and mass-size distribution of near-surface aerosols, made using a ten-channel Quartz Crystal Microbalance (qcm) Impactor for the period October 1998-December 1999 at the tropical coastal station Trivandrum (8.5°N, 77°E), are used to study the response of aerosol characteristics to regional mesoscale and synoptic processes. Results reveal that aerosol mass concentrations are generally higher under land breeze conditions. The sea breeze generally has a cleansing effect, depleting the aerosol loading. The continental air (LB regime) is richer in accumulation mode (submicron) aerosols than the marine air. On a synoptic scale, aerosol mass concentration in the submicron mode decreased from an average high value of ~86 μg m^-3 during the dry months (January-March) to ~11 μg m^-3 during the monsoon season (June-September). On the contrary mass concentration in the supermicron mode increased from a low value of ~15 μg m^-3 during the dry months to reach a comparatively high value of ~35 μg m^-3 during April, May. Correspondingly, the effective radius (R_eff) increased from a low value of 0.15-0.17 μm to ~0.3 μm indicating a seasonal change in the size distribution. The mass-size distribution shows mainly three modes, a fine mode (~0.1 μm); a large mode (~0.5 μm) and a coarse mode (~3 μm). The fine mode dominates in winter. In summer the large mode becomes more conspicuous and the coarse mode builds up. The fine mode is highly reduced in monsoon and the large and coarse modes continue to remain high (replenished) so that their relative dominance increases. The size distribution tends to revert to the winter pattern in the post-monsoon season. Accumulation (submicron) aerosols account for ~98% of the total surface area and ~70% of the total volume of aerosols during winter. During monsoon, even though they still account for ~90% of the area, their contribution to the volume is reduced to ~50%; the coarse aerosols account for the rest.